Field of the Invention
The invention relates to the field of sealing caps comprising an outer metal shell made of aluminum alloy and in particular screw caps that comprise an inner insert typically made of plastic material and threaded. These caps are intended for capping containers, substantially glass bottles that contain alcoholic beverages and in particular wine or spirits.
The invention relates more particularly to a method for manufacturing these caps that makes it possible to obtain a substantially uniform brilliance on the top, or head, and on the “vertical” wall or skirt of the outside of the cap.
Description of Related Art
Sealing caps made of aluminum alloy are typically manufactured in the following way:                A stamping press forms preforms of caps from a strip or sheet also called a format, cut into blanks, varnished on both of their surfaces, of a typical thickness ranging from 0.15 to 0.25 mm without taking the varnish into account. According to the height of the skirt (length of the cap preform), one to three stamping passes may be required.        
The brilliant appearance of the cap is substantially given by the quality of the varnish. In certain applications, rather of high end, the brilliant appearance can be obtained on the final cap by a method of metallization of which the speeds are generally low.                These preforms are degreased in an oven at a high temperature, typically from 180 to 210° C., for a period of time ranging from 3 to 5 minutes, in order to remove the stamping lubricant.        They are then lacquered over their entire external surface and are placed in an over in order to cure the lacquer.        These lacquered preforms are printed on the skirt, typically via silk screening, method known to a person skilled in the art as “Offset”, with a final drying of the inks in an oven.        An overprinting varnish is finally applied in order to protect the printing, said varnish being dried during a passage in the oven.        The preform obtained as such is provided with a seal and/or a threaded interior plastic insert.        
Note that all of the aluminum alloys that are concerned in what follows are designated, unless mentioned otherwise, according to the designations defined by the “Aluminum Association” in the “Registration Record Series” that it publishes on a regular basis.
Problem Posed
The growing interest for aesthetic solutions led the applicant to test the use of strips of aluminum alloy of the grade known to a person skilled in the art as “brilliant” or “grand brilliant”, i.e. having a roughness Ra, measured according to the standard NF EN ISO 4287, typically less than 0.15 μm, and even 0.015 μm in the case of “grand brilliant”, for the stamping of the preforms of caps.
However, after the stamping, the quality of the brilliant appearance remains substantially unchanged on the head of the preform, but is highly degraded on the skirt which has a more or less matte appearance.
This problem is well known in particular in patent EP 1304217 B1 of “Italcoat”, of which the priority application in 2001, which describes it in paragraph [0005] and proposes a relatively complex method for resolving it.
Industrially known solutions are of two types:                Stamping of metal coated with varnish also on the surface corresponding to the outside of the cap preform; it is then the brilliance of the varnish that confers the final brilliance, in general of average quality, even if the base metal is brilliant, due to the fact that the appearance that has become matte of the skirt is visible through transparency.        Metallization on the outside of the preform of the final cap, method generally implemented for caps made of plastic and requiring exceptional brilliance, but this method is slow and expensive and because of this is limited to the field of cosmetics packaging.        
The invention aims to overcome this problem by proposing a method that retains the initial brilliant appearance over the entire surface of the preform, head and skirt, in conditions that are industrially economical for the market of sealing caps for beverage bottles.